The processing of comminuted cellulosic fibrous material, for example, wood chips, to produce cellulose pulp is an energy-intensive process. Treatment temperatures greater than 100.degree. C. and superatmospheric pressures are typical of most commercially-significant chemical treatments of cellulose, for example, the kraft process (also known as the sulfate process). What has made the kraft process so commercially successful is its inherent regeneration of energy from the combustion of waste cooking liquors and the regeneration of cooking chemical from the products of combustion through recausticization. However, due to the cost of energy in the late twentieth century, any further improvement to the energy efficiency of the chemical pulping process is welcome.
In published International application WO 96/32531 (the disclosure of which is hereby incorporated by reference herein) a method of recovering energy from spent cooking liquors by passing the liquors in heat-exchange relationship with cooler liquors, for example, in a steam generating device referred to as a "reboiler", to produce fresh, uncontaminated steam, is disclosed. This method contrasted with earlier methods of recovering energy from spent liquor by using flash evaporation. Such earlier methods inherently entrained undesirable sulfur-bearing compounds, such as hydrogen sulfide [H.sub.2 S], methyl mercaptan [CH.sub.3 SH], dimethyl sulfide [CH.sub.3 SCH.sub.3 ], and dimethyl disulfide [CH.sub.3 SSCH.sub.3 ] as well as other gases, referred to collectively as Total Reduced Sulfur gases or TRS gases. When such steam generated by the flashing of spent cooking liquors was used to pretreat wood chips, the undesirable TRS gases introduced an additional treatment load on the Non-Condensable Gas (NCG) collection system, if present, or introduced the risk of releasing such undesirable, malodorous compounds to the environment.
Published PCT application WO 97/29236 discloses various methods for recovering and utilizing the heat of spent cooking liquor. These include the use of liquor-liquor heat exchangers, reflux boilers and evaporators. However, though this PCT application discloses a process that is similar to the process disclosed in WO 96/32531 in which a source of "clean" steam is provided, this PCT application does not disclose nor suggest the use of a process according to the present invention.
The present invention represents an improvement to the invention in WO 96/32531 by decreasing the temperature of the steam that can be produced by the reboiler or other steam generating device so that more energy in the form of heat can be recovered from the spent cooking liquor and re-used in and around the pulp mill.
In its broadest embodiment the present invention comprises or consists of a method and apparatus for increasing the energy efficiency of a cellulose digester heat recovery system by: (a) introducing a slurry of comminuted cellulosic fibrous material and liquid to a treatment vessel to produce cellulose pulp; (b) treating the material in the vessel with treatment chemical at a temperature greater than 140.degree. C. and a pressure greater than 5 bar gage to produce a treated material and a spent treatment liquor, including sulphur compounds; (c) removing the spent treatment liquid from the vessel at a first temperature; (d) passing the spent liquor in indirect heat exchange relationship with a cooler liquid having a second temperature lower than the first temperature to produce steam (e.g. substantially clean steam) at a first pressure, and at a third temperature higher than the second temperature; (e) increasing the pressure of the steam produced to a second pressure higher (i.e. at least 10% higher and/or by at least about 0.2 bar gage and/or to at least 0.2 bar gage) than the first pressure; and (f) using the steam at the second pressure as needed in the digester system.
In the practice of the method set forth above, (e) may be practiced using a fan or compressor, or an eductor (also known as an ejector) wherein steam at a pressure of between about 3 and 13 bar gage is added to the steam from (d) in the eductor to increase the pressure of the steam from (d). When (e) is practiced so that the second pressure is at least 0.2 bar gage (e.g. 0.4 bar gage or more), (e) is also preferably practiced so that the temperature of the steam is at least 105.degree. C. (e.g. 105-109.degree. C. or more), and then (f) may be practiced to steam comminuted cellulose material being introduced into a feed system for the treatment vessel. In another aspect, (d) may be practiced by introducing as the cooler liquid an essentially sulfur-free liquid, or "clean" liquid, for example, essentially clean water, such that the steam produced is essentially clean steam, for example, containing little or no sulfur compounds. In a further embodiment of this invention, the above invention includes an additional step (g), during step (d), of producing a cooler spent cooking liquor from the spent liquor and using at least some of this cooler spent cooking liquor as the source of cooler liquid in step (d).
Also in the practice of the above method, the spent liquor removed in (c) is typically black liquor at about 140-160.degree. C., and the method further comprises (h) passing the black liquor into indirect heat exchange relationship with a third liquid to recover some of the heat from the black liquor in the third liquid prior to the practice of (d). Typically this is achieved by passing the hot black liquor in heat exchange relationship with one or more liquids in one or more heat exchangers. Typically (d) is practiced using a steam generating device, for example a reboiler; and may further comprise positively cooling the spent liquor after it passes through the steam generating device. Again, this positive cooling may be effected by passing the liquor in heat exchange relationship with one or more liquids in one or more heat exchangers after the practice of (d).
In a preferred embodiment, the positive cooling of the spent liquor after the steam generating device is effected by passing the spent liquor in indirect heat exchange relationship with a fourth liquid to reduce the spent liquor temperature below the prevailing boiling temperature of the liquor (typically around 100.degree. C.) so that the spent liquid may be stored in an unpressurized state, for example, in an unpressurized storage tank, prior to further treatment.
According to another aspect of the invention a method of producing steam having less noncondensible gases than steam produced by flashing black liquor, including sulphur compounds, directly into steam, in a digester system, is provided. The method comprises: (a) Passing hot black liquor, including sulphur compounds, at a temperature of about 120-165.degree. C. from a digester through a heat exchanger. (b) Passing an evaporable liquid to be evaporated through the heat exchanger into heat exchange contact with the hot black liquor so that the evaporable liquid is heated so that it is ultimately evaporated to produce steam having less condensable gases than steam produced by flashing black liquor, including sulphur compounds, directly into steam. (c) Increasing the pressure of steam from (b) by at least 0.2 bar gage. And, (d) using the steam from (c) in the digester system.
In this method, preferably the evaporable liquid used in (b) is substantially free of sulphur compounds; and (b) is practiced to produce substantially clean steam. Step (c) may be practiced using an eductor, fan, or compressor, as described above, and to produce substantially clean steam at a pressure of at least 0.2 bar gage and a temperature of at least 105.degree. C. The method may also comprise (e) passing the hot black liquor into indirect heat exchange relationship with another liquid to recover some of the heat from the black liquor in the other liquid prior to the practice of (b). In a preferred embodiment the noncondensible gases that are reduced are preferably one or more Total Reduced Sulfur (TRS) gases, as described above.
According to another aspect of the present invention there is provided a digester system comprising: A digester having a hot spent treatment liquor outlet. A feed system for feeding comminuted cellulose material to the digester. A steam generating device operatively connected to the spent treatment liquor outlet to use the heat of the spent treatment liquor to produce steam from another liquid, the steam generating device having a steam outlet and a spent treatment liquor outlet. A pressure increasing device, connected to the steam outlet, which increases the pressure of the steam from the steam generating device. And, a utilization device operatively connected to the pressure utilization device to utilize the steam therefrom.
In the digester system the pressure increasing device may be a fan, compressor, or eductor. The system may also comprise an indirect heat exchanger connected between the spent treatment liquor outlet and the steam generating device for recovery of some of the heat from the hot spent treatment liquor before that liquor is passed to the steam generating device; and a flow control valve between the pressure increasing device and the utilization device controlled by feedback from the utilization device, in which case preferably the utilization device comprises the feed system (e.g. a chip bin therein).
It is the primary object of the present invention to provide enhanced recovery and effective use of energy from spent cooking liquor during chemical pulping. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.